what is an aggregate pier?

Aggregate piers are widely accepted by geotechnical and structural engineers to mitigate settlement and increase structural capacity of undocumented fill, compressible soils, unconsolidated granular soils, and many other soil profiles that have low bearing capacities and are susceptible to long & short term settlements. 

Aggregate piers are compacted columns of crushed limestone, recycled concrete, or sand built in lifts.  Each lift is introduced and compacted successively from the bottom of designed depth to rough subgrade elevation.  Each pier extends through problematic soils terminating in a competent bearing layer.  The result is increased soil density, increased soil strength and stiffness, increased bearing capacity, reduced soil compressibility, and increase soil shear strength.

Aggregate piers are installed to support the structural foundation elements of a shallow foundation - mat, continuous, and isolated column footings.  Depending on slab loads and settlement risks relating back to the soil profile, aggregate piers may be installed in a grid pattern under the slab to mitigate cracking and differential settlements between construction joints.

 

What makes an aggregate pier highly valuable in the construction process is not only its quick installation speed, but its positive influence on other parts of the foundation construction process.  Aggregate piers allow the use of traditional/conventional shallow foundation designs, significantly lowering costs compared to otherwise schedule and cost prohibitive deep foundation or over excavation methods. 

How are they installed?

The piers are compacted columns of ¾” stone built in lifts of 1-2ft each.  Each lift is introduced and compacted successively from the bottom of designed depth to rough subgrade elevation.  Each pier extends through problematic soils terminating in a competent bearing layer.

The direct displacement top feed method illustrated below, shows the vibroflot penetrating the soil down to the design depth creating an open borehole creating negligible spoils in the process.  Initial soil penetration begins the ground improvement process by laterally displacing soil.  An open borehole can also be accomplished through pre-drilling.  The vibroflot is retracted from the borehole and aggregate is introduced at the ground surface into the open hole.  The vibroflot is then extended down in the aggregate and compacted.  The aggregate is forced down and laterally outwards into the soil mass.  The process repeats until a continuous compacted column of aggregate is constructed to the design depth up to subgrade elevation.  Onboard instrumentation and computers monitor and record the hydraulic pressure, depth, and GPS location of the vibroflot during installation of every pier. 

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